Many oil and gas wells are drilled into production strata composed of limestone, dolemite and other calcarous materials. Frequently, the production of hydrocarbons from these wells is less than desirable due to the low permeability of the strata which results in poor conductivity to the wellbore. It is common practice to fracture-acidize these formations in an attempt to improve the conductivity. This technique is accomplished by injecting an aqueous acid solution into the wellbore causing a fracture in the production strata. This fracture provides a permeable, conductive channel from the strata to the wellbore.
Hydrochloric acid in concentrations ranging from 3% to 28% is normally used in this stimulation of the carbonate formations. However, in using hydrochloric acid, the reaction of acid with carbonate rock occurs at such a fast rate that the acid is depleted rapidly. Such rapid depletion of acids results in a very limited depth of penetration of acid into the formation. In order to increase the depth of penetration of acid into the formation, acids have been retarded by adding a suitable viscosifying agent, chemical retarders, foaming the acid, emulsifying the acid, or by cross-linking the acid viscosifying agent. Each method of retardation has shown limited use in field applications. Each of these methods of retardation generate fluids of low viscosities and a low degree of viscous stability (viscosities changing with time) especially with increasing reservoir temperature.
It has been found advantageous to initially viscosify the fracture-acidizing fluid. The fluid's viscosity is proportionally related to the created fracture geometry and fracture width so that more viscous fluids will produce longer and wider fractures. In addition, the viscosity of the acid will decrease the reaction rate or retard the acid etching, allowing high strength acid to penetrate deep into the fracture during injection. After the viscous acid is injected into the fracture, the viscosity should diminish allowing the acid to etch the fracture faces. Stable viscosity and control thereof are a problem.
The previous fracture-acidizing fluids have been viscosified with synthetic polymers, derivatives of natural materials such as carboxymethyl cellulose, hydroxypropyl guar or biopolymers such as xanthan gum, and the like. The apparent viscosity of the thickened acid fluids is often low and decreases rapidly in strong acids due to the hydrolysis of the polymer. Synthetic polymers also often precipitate from solution in the presence of high concentrations of calcium and magnesium ions. These percipitants tend to plug the formation which can be detrimental to the recovery of the oil and natural gas from the reservoir. Improved thickened fracture-acidizing compositions are an objective of this invention.